Because of superior resistance to atmospheric corrosion, titanium materials have been used for building roofs and exterior walls exposed to severe corrosive environments in, for example, coastal areas. While approximately ten years have passed since the use of titanium materials as building materials, no case of corrosion has been reported so far. Yet, discoloration unpleasant or offensive to view can happen during long use in some environments. Although discoloration can be controlled by chemically or mechanically reducing the subsurface, low efficiency and high costliness are the problems with roofs and other applications of large areas.
Although the cause of titanium discoloration has not been fully clarified, it has been pointed out that discoloration might possibly result from the adhesion of iron, carbon, silicon dioxide and some other substances in the atmosphere or the development of interference color through the thickness increase of titanium oxide film at the surface of titanium materials.
Japanese Provisional Patent Publication No. 8234 of 1998 discloses a method to reduce discoloration by using titanium materials having surface roughness of not greater than Ra 3 μm and oxide film thickness of not smaller than 20 angstrom. However, the same publication describes nothing about the carbon at the surface and other compositional features.
Japanese Provisional Patent Publication No. 1729 of 2000 discloses use of titanium materials having oxide film thickness of not greater than 100 angstrom and containing not more than 30 at % carbon at the surface. The description says that titanium materials of this type can be obtained by reducing a certain amount of the surface by pickling. However, there is no description of the composition and concentration of the pickling liquid and their influences. No description is given about the influence of fluorine at the surface, too.
Titanium materials are generally pickled with an aqueous solution (of fluonitric acid) containing approximately 10 to 50 g of hydrofluoric acid and approximately 100 to 200 g of nitric acid (approximately 5 to 10 times greater than the concentration of hydrofluoric acid) per liter.
In order to prevent discoloration of titanium materials, the inventors carefully studied influences of surface roughness, oxide film thickness and carbon content on discoloration by conducing surface analyses on discolored roof materials collected from various parts of Japan and accelerated discoloration tests. The investigation revealed that the inventions disclosed in Japanese Provisional Patent Publication No. 8234 of 1998 and No. 1729 of 2000 failed to sufficiently prevent discoloration. No sufficiently effective methods to prevent titanium discoloration in the atmosphere are present.
An object of this invention is to provide titanium materials less susceptible to discoloration that will remain undisfigured for a long time through the control of discoloration that is likely to occur on titanium materials used for roofs, walls and other building materials and methods for manufacturing such titanium materials.
Other objects of this invention are obvious from the following description.